As generally known in the art, cerium oxide (CeO2) is a high functional ceramic material that is widely used in catalysts, phosphors, cosmetics and polishing agents, and has recently been spotlighted as a polishing agent for use in an STI (Shallow Trench Isolation) process of a semiconductor device and as an optical glass polishing agent. Particularly, in using cerium oxide (CeO2) powder as a polishing agent for CMP slurry, there is an earnest need for synthesis of uniform powder with spherical particle shape and small particle size, but a solid-phase process in high temperature with difficulty in controlling particle size and dispersibility due to its high process temperature is the only method so far known to be industrially applicable to the preparation of cerium oxide (CeO2) powder. In addition, although various wet processes, such as a coprecipitation process, a hydrothermal process, an emulsion process, etc., are relatively easy to control particle size and shape, any research showing that the wet processes result in the successful synthesis of single crystalline cerium oxide (CeO2) powder with an average particle size of 30 nm or greater has not been reported. Thus, much difficulty has been experienced in industrially utilizing cerium oxide (CeO2) powder.
In recent years, there has been an increased interest in research on ceramic powder synthesis and its commercial applications, which was sparked by the recognition that the conventional method of obtaining powder by comminuting minerals imposes restrictions on making the most of superior properties of ceramics. With regard to this, since wet processes, such as a precipitation process, a sol-gel process, a hydrothermal process, and so forth, make it possible to develop new characteristics of ceramics and to obtain high value-added ceramic products by remedying such a drawback of the existing pulverization method, active research on the wet processes is being pursued.
Particularly, much research focuses on the precipitation process and its commercial applications due to its ability to grow crystal particles and control particle size and shape in a solution phase where a much lower temperature than that in a solid-phase reaction prevails, as well as due to its easy control of particle size and shape, which is an advantage common to all the wet processes. Although the wet processes including the precipitation process are easy to prepare fine particles because they are conducted in a build-up manner in which small nuclei grow into larger particles, they have difficulty in preparing particles with large particle size and high crystallinity.
Many researchers attempted to overcome this problem, for example, by simply performing crystal growth after controlling initial starting particle size by use of a seed, carrying out a high-temperature and high-pressure reaction process in a supercritical state above the critical point of water, or using a high concentration of acid/base for increasing solubility, but all of such attempts failed in commercial utilization. Among others, the supercritical fluid process using supercritical water requires equipment capable of a high-temperature and high-pressure reaction, which is extremely costly and consumptive of parts, and has difficulty in controlling reaction conditions, so its commercial applications is still a long way off, even though continued research thereon is being pursued.
In an example of research on the preparation of cerium oxide (CeO2) powder, Matijevic et al. reported that hexagonal plate-like and spherical cerium oxide particles can be obtained from starting materials consisting of Ce(SO4)2·4H2O, (NH4)4Ce(SO4)4·2H2O, (NH4)2Ce(NO3)6 and others by sealing the starting material in a Pyrex tube, heating the sealed material at a constant temperature to thereby precipitate cerium hydroxide, and then calcining the precipitate at a temperature of about 600° C. (Wan Peter Hsu, Lena Roannquist, Egon Matijevic, Preparation and Properties of Monodispersed Colloidal Particles of Lanthanide Compounds. 2. Cerium(IV), Langmuir, 4, 31-37 (1988)).
In another example of research on the preparation of cerium oxide (CeO2) powder, E. Tani et al. prepared cerium oxide powder with an average particle size of 100 μm by precipitating hydrate from starting materials of cerium nitrate and NH4OH, and firing the precipitated hydrate together with various additives at a temperature of about 500 to 600° C. (E Tani, M. Yoshimura, S Somiya, Crystallization and crystal growth of CeO2 under hydrothermal consitions, Journal of the Materials Science Letters, 1, 461-462, (1982)).
In yet another example of research on the preparation of cerium oxide (CeO2) powder, Takuya Tsuzuki et al. prepared uniform nanosized cerium oxide powder from starting materials of cerium chloride (CeCl3) and NaOH by using a mechanochemical process and a calcination process. When cerium chloride and NaOH with NaCl added thereto were comminuted by steel balls in a primary comminution process, cerium hydroxide was synthesized through a mechanochemical reaction. Also, the synthesized cerium hydroxide was calcined at a temperature of 500° C. or greater to thereby synthesize nanosized spherical cerium oxide. However, the synthesis of cerium oxide powder by such a mechanochemical process has a problem in that a large quantity of Na, which is a fatal contaminant in semiconductor processes, is contained in the synthesized cerium oxide powder, and thus a separate cleaning process is inevitable. Also, due to agglomeration and crystallization according to the calcination process, a large amount of energy is required for the comminution into nanosized particles. Therefore, with regard to its commercialization and application to a CMP process, there are still many problems to be solved (Takuya Tsuzuki, Paul G, McCormick, Synthesis of Ultrafine Ceria Powders by Mechanochemical Processing, Journal of the American Ceramic Society, 84(7), 1453-58, (2001)).
Cerium oxide powder prepared by the above-mentioned precipitation process has a limitation on crystal growth and thus an increase in particle size. On account of this, when cerium oxide powder with a particle size of not less than specific size is to be obtained, there is a problem in that a solid-phase process must be carried out in parallel in such a manner that hydrate corresponding to an intermediate material of cerium oxide is prepared, and then is calcined by heat treatment at a high temperature.